[Bug c/96788] "integer constant is so large that it is unsigned" warning is incorrect

[pascal_cuoq at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96788

Pascal Cuoq <pascal_cuoq at hotmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |pascal_cuoq at hotmail dot com

--- Comment #6 from Pascal Cuoq <pascal_cuoq at hotmail dot com> ---
Joseph Myers wrote:
> The warnings are attempting to cover both the C90 
> case where a decimal constant too large for signed long can be unsigned 
> long, and the case of a constant too large for intmax_t.

In the case of the constant 10000000000000000000, each of the two kind of
warnings, “ integer constant is so large that it is unsigned” and “this decimal
constant is unsigned only in ISO C90” can be misleading.

Consider the compilation unit:

int x;

void f(void) {
    x = -10000000000000000000 < 0;
}

1/ Misleading warning “this decimal constant is unsigned only in ISO C90”

Compiler Explorer link: https://gcc.godbolt.org/z/4Eze1f

Using GCC 10.2 targeting x86, the compilation options “-O -m32 -std=c89” make
GCC compile f as below, and emit the warning “this decimal constant is unsigned
only in ISO C90”

f:
        movl    $0, x
        ret

Actually, changing the C dialect to C99 does not make the constant not
unsigned, since GCC emits the same assembly code for f (setting x to 0) with
“-O -m32 -std=c99”

2/ Misleading warning “integer constant is so large that it is unsigned”

Compiler Explorer link: https://gcc.godbolt.org/z/MGjn5G

Using GCC 10.2 targeting x86, the compilation options “-O -m64 -std=c99” make
GCC compile f as below, and emit the warning “integer constant is so large that
it is unsigned”.

f:
        movl    $1, x(%rip)
        ret

The constant is not unsigned. If it were, the function f would not set x to 1.